public string NextResponse(Random rng)
{
switch (Responses.Count)
{
case 0:
return(null);
case 1:
return(Responses[0]);
default:
int pickedIndex = CurrentIndexes[NextIndexIndex];
string response = Responses[pickedIndex];
++NextIndexIndex;
if (NextIndexIndex >= CurrentIndexes.Count)
{
NextIndexIndex = 0;
CurrentIndexes.Shuffle(rng);
// make sure the final index of last round isn't the same as the first index of this round
// this prevents an item appearing twice in succession (when a shuffle happens in between)
if (CurrentIndexes[0] == pickedIndex)
{
// swap once more
int otherIndex = rng.Next(1, CurrentIndexes.Count);
int temp = CurrentIndexes[0];
CurrentIndexes[0] = CurrentIndexes[otherIndex];
CurrentIndexes[otherIndex] = temp;
}
}
return(response);
}
}
private void ObjectInvariant()
{
// number of responses equals number of indexes
Contract.Invariant(Responses.Count == CurrentIndexes.Count);
// indexes are a permutation of the range 0..Count-1
Contract.Invariant(CurrentIndexes.OrderBy(i => i).SequenceEqual(Enumerable.Range(0, CurrentIndexes.Count)));
// NextIndexIndex is within 0..Count-1
Contract.Invariant(NextIndexIndex >= 0 && NextIndexIndex < CurrentIndexes.Count);
}
